1. Field of the Invention
The present invention relates to a technique for forming halftone dots used to represent an original image of gray scale.
2. Description of the Background Art
In many cases where printing data or printing plate data are made from grayscale images, creation of halftone dots, as one of halftoning method, is used. Among well-known types of dot shape are square, chain, elliptical one, round one or the like. Halftone dots to make gradation with the number of dots unchanged and the size of dots changed are referred to as “AM halftone dots”.
The AM halftone dot, however, has the following problems of: a) causing moire when an image having a periodic pattern is represented by halftone dots, b) causing a rosette pattern (moire pattern) in multicolor printing, c) having difficulty in avoiding moire when four or more colors are superposed in printing and so being not suitable for high quality color printing, and so on. The above problems a) and b) can be solved by increasing screen ruling serving as reference for writing of halftone dots (in other words, by fining the halftone dots), but in this case, there arises new problems of: d) degrading reproduction in printing of minimum writing dots in a highlight area and a shadow area, and e) making banding noticeable (noticeable lines in a scan direction caused by an influence of an output equipment, not due to halftone dot structure, e.g., caused by non-uniformity in dot size in multichannel writing or incomplete arrangement of swath (band written in a main scan) with respect to a subscan direction.
The problem d) can be solved by increasing the minimum dot size by using threshold matrix shown in FIG. 6.24 of pp. 168 to 170 of “Fundamentals of Electronic Imaging Systems” by F. W. Schreiber, U.S.A. the third edition, Springer-Verlag New York, May 1, 1993, or by using methods disclosed in Japanese Patent Application Laid Open Gazette No. 10-145593 or patent Publication No. 2736048 to leave relatively large dots in the highlight area and the shadow area.
On the other hand, for creating halftone dots, a method termed “FM screening” where gradation is made by changing the number of dots of fixed size which are appropriately arranged (at random but not at extremely high or low density) is used in some cases. The FM screening is effective for the above problems a) to c). patent Publication No. 3427026 discloses a method of connecting dots at random and Japanese Patent Application Laid Open Gazette No. 11-177821 discloses a method of decreasing the number of dots in the highlight area. These methods are effective for the above problems a) to d) to be solved. In patent Publication No. 3427026 and Japanese Patent Application Laid Open Gazette No. 1 1-177821, the centers of dots are arranged at random and the shapes of the halftone cells each serving as a unit for formation of a halftone dot are obtained by e.g., using a Voronoi diagram (see “separate volume of “bit”, Computational Geometry and Geographic Information Technology”, supervised by Masao Iri, published by Kyoritsu Shuppan Co., Ltd., Sep. 10, 1986, pp. 163 to 168).
For the above problem e), since banding becomes noticeable when superposition is made in printing, it is possible to reduce banding (noticeable lines) in multicolor printing by using different channels for color components to perform writing to one portion in multichannel writing shown in U.S. Pat. No. 5,182,990.
The halftone dots created by FM screening, which are fine in structure, are likely cause the banding (the above problem e) as compared with the AM halftone dots and the AM halftone dots make banding noticeable when they are fine. There is actually no effective measure to solve the problem of banding. The methods of Patent Publication No. 3427026 and Japanese Patent Application Laid Open Gazette No. 11-177821 are effective for a beat pattern which appears in a specific direction irrelevant to the scan direction, by interference between the halftone structure and the number of channels for printing or writing but have room for improvement in solution of banding which is caused in the scan direction, being independent of the halftone structure.
The method of U.S. Pat. No. 5,182,990 is effective for multicolor printing but not a radical solution for banding since it has no measure to solve banding for each color component.